1. Field of the Invention
The present invention relates to compilers for computer systems. More specifically, the present invention relates to a method and an apparatus that dynamically compiles byte codes into native code to facilitate faster program execution.
2. Related Art
The exponential growth of the Internet has in part been fueled by the development of computer languages, such as the JAVA™ programming language distributed by Sun Microsystems, Inc. of Palo Alto, Calif. The JAVA programming language allows an application to be compiled into a module containing platform-independent byte codes, which can be distributed across a network of many different computer systems. Any computer system possessing a corresponding platform-independent virtual machine, such as the JAVA virtual machine, is then able to execute the byte codes. In this way, a single form of the application can be easily distributed to and executed by a large number of different computing platforms.
When an application is received in platform-independent form, it can be interpreted directly through an interpreter, or alternatively, it can be compiled into machine code for the native architecture of the computing platform. Machine code executes significantly faster than interpreter code. However, compiled code occupies considerably more space than interpreter code. Hence, the determination of whether or not to compile interpreter code depends on the relative importance of memory space to execution speed.
Some “mixed-mode” computer systems support execution of both compiled code and interpreter code. Determining which methods to compile and which to leave to be executed by the interpreter can be a difficult problem. This is especially true for constrained memory systems, such as pocket-sized computing devices, which have limited memory space to accommodate compiled methods, and which typically do not provide swap space in secondary storage for the virtual memory system. This is not as much of a problem in larger computer systems, which have more memory to store compiled methods, and which can rely on the virtual memory system to migrate unused compiled methods to swap space in secondary storage.
In many cases, statistical information gained through special profiling sessions can be used to determine which methods to compile. However, systems that compile methods based on profiling information operate on assumptions that might not hold true from situation to situation. In some situations a method might be invoked numerous times, and in other situations, such as during profiling, it might not be invoked at all. In addition, profiling requires the methods to be executed to gather statistical information, which requires additional time. Furthermore, profiling code takes up valuable space.
Another possible method for determining which methods to compile is to use invocation counters. Each method can have its own invocation counter that is incremented each time the method is executed. A method is compiled when its corresponding invocation counter exceeds a predetermined value. However, this technique has drawbacks: the invocation counters take up space; infrequently used methods will eventually be compiled; and executing a method in interpreted mode until the invocation counter reaches the predetermined value can consume a significant amount of computational time.
Hence, what is needed is a method and an apparatus for dynamically compiling byte codes into native code without the problems of the above-described techniques.